This application is a continuation of U.S. patent application Ser. No. 12/126,485, filed May 23, 2008, now U.S. Pat. No. 8,487,747, and is herein incorporated in its entirety.
Radio frequency identification (“RFID”) is a term for technologies that use radio waves to identify objects. Specifically, RFID technologies perform automatic identification of objects through storing and remotely retrieving data using RFID tags. Typically, a unique identifier that identifies these objects, as well as additional information, is stored on a microchip that is attached to an antenna within the RFID tag. The antenna enables the chip to transmit the identification information to an RFID reader. Accordingly, the RFID reader converts the radio waves reflected back from the RFID tag into digital information that computers can receive and process on behalf of user applications.
A common problem for a tracking system utilizing RFID technologies is to determine the direction in which an RFID tag approaches and passes by the RFID reader. For example, it may be difficult to determine whether a retail item is entering or exiting a storefront, or whether a vehicle is return to or leaving a location. RFID portals may be placed at the entry points of such locations, wherein a group of RFID readers may track the RFID tags. However, conventional RFID portals simply monitor the time in which one of the RFID readers senses the RFID tag. Specifically, if a first RFID reader senses the RFID tag at time (x) and a second RFID reader sense the same tag at time (x+1), it is assumed that the tag is moving from the first RFID reader towards the second RFID reader. However, this conventional system relies on the accuracy and the strength of signal of the RFID readers. Such a system may be greatly compromised in a confined area.